In order to prevent damage to the cutting unit by metal parts such as e.g., paper clips, staples or the like introduced into the paper feed, some file shredders nowadays have a metal detector. This metal detector usually comprises a sensor device which reacts to metal introduced into the paper guide and switches off the cutting unit and/or the paper feed of the file shredder before the metal parts can come into contact with the cutting elements of the cutting unit.
The mode of operation on which the corresponding metal-detecting sensor device is based is hereby based on the methods generally used for detecting/recognizing metal, such as, for example, pulse measurements based on induction or alternating current measurements.
Due to the small size of the metal object to be detected, and the resulting low signal level, in practice the problem arises with the file shredders according to the prior art that the signal-to-noise ratio is very small, so that a reliable detection of small metal parts is not given.